The present invention refers to glass-ceramic consisting of the mixtures (I) or (II):
ZrO2xe2x80x94SiO2xe2x80x94Mexe2x80x3Oxe2x80x83xe2x80x83(I)
SiO2xe2x80x94Me2xe2x80x2xe2x80x3O3xe2x80x83xe2x80x83(II)
wherein in mixture (I):
Mexe2x80x3=Ca, Ba, Mg, Zn or mixtures thereof:
and the percentage, in weight, for each component is:
ZrO2 5-25%
SiO2 45-75%
MeOxe2x80x3 15-45%
and in mixture (II)
Mexe2x80x3O is as above defined;
Mexe2x80x2xe2x80x3=Al, B or mixtures thereof
and the percentage in weight for each of the above said components is:
SiO2 30-65%
Me2xe2x80x2xe2x80x3O3 5-25%
Mexe2x80x3O 5-40% characterized in that the glass ceramics are obtained in the form of powders.
It is known that glass is an amorphous material obtained by melting of crystalline compounds followed by cooling down of the melted mass. On the contrary glass-ceramics (hereinafter indicated as GC) are vitreous systems that, when brought to a temperature T1 higher then their glass transition temperature Tg, present the formation of crystal nuclei (homogeneous or heterogeneous) with following crystal growth. Porcelain stoneware (also defined as ceramic body having absorption  less than /=0.5%, according to ISO 13006 annnex B1A) is a ceramic material prepared starting from natural crystalline products which, submitted to a syntherization process, partially melt and are transformed into new crystalline phases. This products are prepared starting from a mixture of clay minerals, fondents and possibly eutectic promoters. Glaze is a ceramic product consisting of fondents and silica based glasses which are grinded in granules of the wanted dimensions and thereafter applied on the appropriate substrate and heated so that the grinded granules melt (totally or partially) covering the substrate surface.
All the above said products, having a vitreous surface, confer impermeability and higher physico-chemical properties (better resistance to chemical agents, abrasion etc.) to porous substrates. Moreover they play a very important role as aesthetic materials due to the use of lead based fondents, opacifiers (represented by dispersions of, for example, tin oxide or zirconium silicate which are added in the form of crystals having a determined granulometric dimensions) and coloured pigments.
In JP-A-62 072539 a crystallised glass is described which is obtained by molding glass containing SiO2 and ZrO2 as the essential components and crystallising it at 750-1300xc2x0 C. to deposit tetragonal zirconia in the glass.
However the known materials do not completely satisfy the market needs in so far as the physico-chemical properties are concerned and also their opacization (and the consequent white index) is not always satisfactory giving final products which are aesthetically not suitable for the appropriate use; moreover the known processes for the production of glass-ceramics involve high costs and waste of energy.
The invention solves the above said problems thanks to new glass-ceramic having the composition reported above and characterized in that the glass ceramics are obtained in the form of powders.
The glass-ceramic according to the invention can be used as such in order to obtain ceramics or can be added to the materials usually employed for preparing porcelain stoneware or glazes.
The use of the glass-ceramics according to the invention allows (thanks to their xe2x80x9cin situxe2x80x9d crystallization) to confer to glaze an exceptional opacization which results in an higher white index (WI) compared to that obtained with the normal opacifiers added to glass, moreover, thanks to the chemical stability and high mechanical resistance of the crystalline phase recrystallized on the glaze surface, they confer to the substrate physico-chemical properties higher then those of the traditional glazes.
The glass-ceramics according to the invention allow also the preparation of ceramics directly from the melted mass which can be shaped in the desired form or the preparation of the wanted ceramic items by tape casting of slurries or hot and cold pressing of the powders.
In the following TABLE 1 preferred glass-ceramic according to the invention are reported (the percentage of the components is given in weight):
The glass-ceramics according to the present invention can be prepared according to a process which is substantially similar to the one followed for the production of porcelain stoneware consisting in the grinding/mixing/pressing (and following firing of the pressed materials) of powders with the difference that in this case the powders consists only of glass (i.e. they lack the crystalline starting materials used for the production of porcelain stoneware).
However, it is essential that the firing step is performed according to well defined and controlled thermic cycles in order to develop the wanted properties.
In particular, once the Tg (transition temperature) and the Tc (crystallization temperature/s) of the material are determined (according to usual methods) the thermic cycle must be performed as follows:
starting from room temperature the powder mixture is heated increasing the temperature by 10xc2x0-30xc2x0 C. per minute up to 350xc2x0 C. (in order to eliminate the organic impurities according to usual processes) thereafter the heating temperature is increased by 10xc2x0-30xc2x0 C. per minute up to the Tg maintaining the temperature at this value for 0-120xe2x80x2, thereafter the temperature is increased by 10xc2x0-30xc2x0 C. per minute up to the Tc were it is maintained for 0-several hours, for example up to seven hours, preferably for 0-4 hours, and possibly increased by 10xc2x0-30xc2x0 C. up to the next Tc and so on up to the final Tc; once completed the heating (i.e. once the highest Tc has been reached) the mass is cooled down to room temperature.
As above said the glass-ceramics according to the invention can be added to the materials normally used for the production of porcelain stoneware; the adding of the glass-ceramics according to the invention allows the production of ceramics via the process normally used for the production of porcelain stoneware this resulting in an important saving of energy.
For example a glass-ceramic according to the invention is added to a traditional mixture of starting materials for the production of porcelain stoneware; the mixture is loaded in a Alsing ball-mill (continuous or discontinuous) and grinded in the presence of water to give a slurry which is dried in an atomizer forming hollow grains. The grains are shaped in the form of tiles by traditional pressing. The tiles are dried and fired in a traditional furnace at 900xc2x0-1230xc2x0 C. giving the final product. The obtained tiles are constituted of syntherized and crystallized materials and a residual vitreous phase as demonstrated by mineral and X-ray analysis of the file surface.
Preferably the glass-ceramics according to the invention are added to the traditional materials in a quantity comprised between 5% and 65% (in weight). In TABLE 2 it is reported (expressed in metal oxide percentage in weight) an example of the composition of a mixture of starting materials which is added to the glass-ceramics of the present invention in the above reported quantities:
In this connection it was surprisingly found, and it is another object of the present invention, that also other glass-ceramics, already known xe2x80x9cper sexe2x80x9d, can be added to the traditional starting materials for production of porcelain stoneware giving similar advantageous results.
Examples of already known glass-ceramics (a-d) which can be used for the above said purpose are given in following TABLE 3 (the percentage is expressed in weight).
The glazes can be produced via the processes traditionally used for the production of glazes. A glass-ceramic according to the invention and a starting material usually employed for the production of ceramic glazes (which are essentially the same reported for the preparation of porcelain stoneware with the addition of fondents like frits or borates or lead oxide etc.) were loaded in the appropriated proportions in a Alsing ball-mill in the presence of water and grinded to obtain a so called xe2x80x9cglazexe2x80x9d which was applied by airbrush, threading die, bell or serigraphy on a traditional substrate, crude or biscuited, obtained by pressing of atomized or dry-grinded powders. The glaze, dried and/or granulated, can be deposited on the substrate surface by falling and fixed on the surface with appropriated ligants. The substrate is fired at 900xc2x0-1230xc2x0 C. in a quick- or tunnel-furnace (continuous or discontinuous) giving the final product in which the glass-ceramic has induced a controlled crystallization. The glazed tiles so obtained present therefore a vitreous and a crystallized part which confer a very well defined microstructure as shown by SEM and X-rays diffractometry.
Preferably the glass-ceramics according to the invention are added to the traditional materials in a quantity comprised between 5% and 60% (in weight).